There are several respiratory virus's that can infect chickens, some of them are chronic (not cured - the bird remains a carrier), and treatment may help with symptoms, but the symptoms will recur from time to time, usually with some kind of stress. The birds that are carriers will continue to spread the virus to new birds in your flock. If you have access to a vet that can do some testing for you (often either a blood test or a swab and culture) that would be the best way to know exactly what you are dealing with and then could decide how to proceed. Or if you lose one and send it for necropsy, they will do labs and tell you what it is. If you have more specific symptoms you can describe then someone here with more experience with respiratory diseases can help you. Treatment is often Tylan, which depending on where you are, may be available at a feed store.
Here is more info that may help: https://hencam.com/faq/respiratory-diseases/ http://www.veterinaryworld.org/2008/July/Common Respiratory Diseases of Poultry.pdf
We just experienced/are experiencing our flock having MG, which is one of the respiratory diseases you'll find above. All of the chickens seemed to have different symptoms. Some just sneezed, the worst two had gurgling/snotty noses, and all had swollen eyes. One had swollen watery. bubbly eyes. We gave them Tylan 50 orally, 1cc for a 5lb chicken 3x a day is what we were told, and I also gave 1cc of coilloidal silver orally as well. I put Vetrx on their nostrils 2x a day. It's been about a week and they are looking so much better. but I did have to give water manually to the two worst presenting chickens and I kinda force fed them too (just put pellets in their mouth, they would then eat). From my understanding the respiratory disease will always stay with them. Many people seem to choose to cull the entire flock. We don't plan to sell our birds, because they could infect other flocks and that would be a terrible thing to do. If your birds have MG, they will pass that on as they lay so if you want chicks from them, please don't sell them because they will infect other flocks.
What is your location? (state/country)
When a bird dies of unknown cause, it is best to have a necropsy and lab work so you know how to proceed.
There are so many things that could cause those symptoms. There are no medications for viruses, just supportive care.
This time of year, in the northern hemisphere, many people seem to experience respiratory illness in their chickens. I firmly believe it is because they are trying to keep them warm rather than keeping the ventilation wide open. Fresh air for chickens is more important than food and water - not warmth.
However, now that your birds are sick, they do need some warmth while they recover.
Since many of the respiratory diseases look alike, I would agree to get some PCR testing, or a necropsy by the state vet on any chickens that die. Closing your flock to new birds or any leaving your flock should be practiced.